Vertical LEDs have a top electrode and a bottom electrode, such as a top anode and a bottom cathode. Current flows vertically through the LED layers to cause photons to be emitted from the active layer.
It is well-known to provide a phosphor on the light-emitting side of the LED die to wavelength-convert the LED light. For example, the phosphor layer may be a YAG phosphor that emits yellow light when energized by a blue light, and the active layer may emit blue light. Some of the blue light leaks through the phosphor layer to combine with the yellow light to create white light. Typically, the phosphor layer is composed of ceramic phosphor particles in a transparent dielectric binder, such as silicone.
The phosphor layer must allow the LED's top metal electrode to be exposed so that a wire can be bonded to the top electrode. This can be done by etching the phosphor layer over the electrode. The top electrode must be made small so as not to block a significant portion of the light, such as a narrow metal ring around the top perimeter of the LED, or patterned to have narrow fingers, etc. A transparent conductor layer may be deposited between the phosphor layer and the LED semiconductor layers to help spread the current laterally from top metal electrode.
One problem with such a configuration is that the interface of the phosphor layer and the transparent conductor layer may result in total internal reflection (TIR) back into the LED, resulting in some absorption by the semiconductor layers. The transparent conductor layer also attenuates the light and thus lowers the conversion efficiency. Another problem is that etching the phosphor layer to expose the top electrode reduces the amount of phosphor available for wavelength conversion and also results in lower color uniformity across the LED. Etching the phosphor also wastes the phosphor and adds an extra step. Further, the top metal electrode blocks some of the LED light.
What is needed is a technique for wavelength-converting LED light using a phosphor that does not have the drawbacks of the above-described devices.
It is known to form an electrically conductive phosphor layer for light-emitting field emission devices (not LEDs), where opposing transparent conductive plates have a high voltage applied to them, and a conductive phosphor layer lines one or both conductive plates. Such a conductive phosphor and field emission device are described in United States publication US 2012/0248967, incorporated herein by reference. The conductive phosphor is pre-formed as a paste using carbon nanotubes, phosphor powder, and a suitable organic vehicle. The paste is then spread on the conductive plate. The organic vehicle is removed by heat during curing. Electrons are drawn by the electric field and energize the phosphor to emit light. However, such phosphors are the types used to convert high energy electrons to light (such as used in CRTs) rather than wavelength-convert visible light and are thus very different from phosphors used in LEDs. Further, the structure is such that there is no blue or visible light that leaks through the phosphor layer that combines with the phosphor light to produce the desired overall light. Further, the phosphor layer is relatively thick and dense to convert a maximum amount of the high energy electrons to photons, making it unsuitable for use with an LED. Accordingly, such conductive phosphors are solely for use in a field unrelated to wavelength conversion for LEDs.